L390 vs L415 – Composition, Heat Treatment, Properties, and Applications

Introduction

Engineers, procurement managers, and manufacturing planners frequently face trade-offs when selecting a steel grade: higher strength often conflicts with ease of fabrication and welding, while better weldability can limit achievable peak strength or wear resistance. L390 and L415 are paired in many selection conversations because they occupy adjacent positions in the spectrum of low-alloy structural and tool-type steels where designers balance delivered strength against fabrication convenience.

The primary practical distinction between these two grades is a trade-off between maximum achievable strength/hardenability and ease of welding/fabrication. L415 is generally specified when higher through-thickness strength and hardenability are required, while L390 is often preferred where weldability, toughness, and simpler heat treatment are prioritized. Because naming conventions and exact chemistries can vary by standard or supplier, users should confirm mill certificates and applicable standards for any specific purchase.

1. Standards and Designations

• Common standards to check for both grades: EN (European Norms), ASTM/ASME (American), JIS (Japanese), and national standards such as GB (China). Not all standards use the exact L390/L415 labels; these may be commercial or trade names mapped to equivalent standard numbers.
• Classification:
• L390 — commonly treated as a low-alloy structural or engineering steel; sometimes used in knife/tool/shaping applications where balanced toughness and moderate strength are required.
• L415 — typically a higher-strength low-alloy steel or a higher-hardenability variant used where greater tensile/yield strength or deeper hardening is needed.
• Type: Neither is a stainless grade (unless explicitly designated as such in a supplier spec); both are low-alloy or microalloyed steels rather than conventional tool steels or stainless steels. Confirm whether the grade in question is an alloyed structural steel, a tool steel variant, or a special product from a particular mill.

2. Chemical Composition and Alloying Strategy

The following table summarizes typical alloying strategy qualitatively. Exact compositions vary by specification and manufacturer; consult the actual material specification and mill test certificate for procurement or design.

How alloying affects properties: Elements that raise hardenability (Cr, Mo, Mn, B, and sometimes Ni) allow deeper martensitic transformation during quenching and thus higher through-thickness strength in thicker sections. Microalloying elements (V, Nb, Ti) enable fine-grain strengthening and good toughness without very high carbon, improving strength-to-toughness ratio. However, higher hardenability and carbon equivalents generally reduce weldability and increase preheat/interpass requirements to avoid cracking.

3. Microstructure and Heat Treatment Response

• Typical microstructures:
• L390: after normalized or quenched-and-tempered cycles, L390 tends to show a tempered martensitic or bainitic matrix with relatively fine prior-austenite grains when microalloyed. This yields balanced toughness with moderate strength.
• L415: tends to form a higher fraction of martensite or lower-temperature bainite after quench, especially in thicker sections or when quenched to increase hardness. Prior-austenite grain size control and tempering are critical to obtain acceptable toughness.
• Heat-treatment routes and effects:
• Normalizing: both grades benefit from normalizing to refine prior-austenite grain size; L390 responds well with improved toughness. L415 normalized will show higher strength after subsequent tempering.
• Quench & temper: primary route to obtain high strength. L415 typically requires more aggressive quench media or slower tempering to reach design strengths; tempering is necessary to restore toughness.
• Thermo-mechanical processing: controlled rolling plus accelerated cooling can produce fine bainitic or martensitic-bainitic microstructures with high strength and good toughness, often enabling better weldability than heavy quench/temper cycles.
• Practical note: As hardenability increases, post-weld heat treatment (PWHT) and preheat requirements become more significant to avoid hydrogen-assisted cold cracking.

4. Mechanical Properties

The table below compares expected property tendencies. Actual values depend on heat treatment, product form, and supplier. Use mill test reports for design verification.

Explanation: L415’s higher alloy content and/or carbon equivalent allow designers to reach higher tensile and yield strengths, but these gains often come at the expense of ductility and weldability. L390 emphasizes a balance—reasonable strength while maintaining better toughness and deformability.

5. Weldability

Weldability is influenced by carbon content, carbon equivalent (hardenability), and microalloying elements. Two common predictive formulas are useful to interpret relative behavior:

• IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• Pcm for cold cracking susceptibility: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation (qualitative): - L390: lower-to-moderate $CE_{IIW}$ and $P_{cm}$ values generally indicate easier welding with standard consumables and less stringent preheat. Lower carbon and restrained alloying simplify joint design and reduce required PWHT. - L415: higher alloying and carbon raise $CE_{IIW}$ and $P_{cm}$, increasing susceptibility to hydrogen-assisted and hardening-related cracking. Welding L415 often requires controlled preheat, lower hydrogen consumables, interpass control, and sometimes PWHT.

Practical guidance: For both grades, follow supplier welding procedures; perform PWHT if specified. Use hydrogen-controlled electrodes or filler metals matched to desired strength and ductility. Where welds must be carried out by usual shop practice with limited preheat capability, L390 will usually be the safer choice.

6. Corrosion and Surface Protection

• Neither L390 nor L415 is inherently stainless; corrosion resistance is typical of carbon/low-alloy steels and requires surface protection in exposed environments.
• Common protections:
• Hot-dip galvanizing, zinc spray, or metallizing for atmospheric corrosion protection.
• Paints, epoxy coatings, or powder coat systems for architectural or mild exposures.
• Cathodic protection and specialized coatings for marine or highly corrosive environments.
• PREN (Pitting Resistance Equivalent Number) is not applicable unless a grade is explicitly stainless. For reference, PREN is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ But this index applies only to stainless alloys; L390/L415 should be treated with conventional carbon-steel protection strategies.

7. Fabrication, Machinability, and Formability

• Cutting and machining:
• L390: typically easier to machine in normalized/tempered condition; lower hardness and carbon aid tool life and throughput.
• L415: machining can be more challenging if supplied in high-hardness or quenched condition; pre-machining in softer condition or using carbide tooling and appropriate feeds is recommended.
• Forming and bending:
• L390: better formability, higher allowable bend reductions before cracking in normalized or tempered conditions.
• L415: forming is more limited when high-strength temper is present; cold forming may require annealing or specialized tooling.
• Welding and post-weld finishing:
• L390: easier to grind, dress, and finish after welding.
• L415: may require additional PWHT and careful grinding to avoid tempering effects and to maintain properties.

8. Typical Applications

Selection rationale: choose L390 for jobs that prioritize fabrication, toughness, or where complex welding is unavoidable. Choose L415 for designs demanding higher static strength, wear resistance, or greater load-bearing capacity and where the fabrication environment can accommodate stricter welding and heat-treatment controls.

9. Cost and Availability

• Relative cost: L415 is usually more expensive on a per-ton basis when alloy content and processing (e.g., specialized heat treatment) are higher. L390 tends to be more cost-effective for applications where ultra-high strength is not required.
• Availability by product form: both grades are commonly available in plate, bar, and forged forms from specialty mills and service centers, but lead times vary by regional demand and whether a specific heat treatment or certification is required. Standard stock-keeping tends toward more common structural alloys; specialized high-strength variants may require mill runs or heat treatment delays.
• Procurement tip: specify required heat treatment, hardness or mechanical targets, and required material tests (UT, MT, PMI, MTC) up-front to avoid price and lead-time surprises.

10. Summary and Recommendation

Summary table (qualitative):

Recommendation: - Choose L390 if you need a balanced engineering steel with relatively good weldability, easier machining and forming, and sound toughness for welded structures or components produced in shop environments with standard welding procedures. - Choose L415 if your primary requirement is higher tensile or yield strength, deeper hardening in thicker sections, or greater wear resistance and the manufacturing plan can accommodate more restrictive welding procedures, preheat/PWHT, and potentially higher material cost.

Final note: The terms L390 and L415 can be used differently across suppliers and standards. Always confirm the exact chemical composition, mechanical property requirements, and specified heat-treatment route in the material specification and mill test certificate before final design or procurement.